There are many different types of nuclear reactor transient events. For example, the load on a turbine driven by steam from a nuclear power plant may be removed from the turbine by any one of a number of events, causing a short in the electric power transmission lines. Should such transient event occur, typically the stop valves to the turbine are closed, shutting down the delivery of steam to the turbine. The nuclear reactor, however, is still producing full power. To control the reactor in view of the transient, the control rods are driven into the reactor and valves are opened to bypass steam to a condenser. Safety considerations, however, require preparation for failures in the system, including, for example, those where the condenser bypass valve cannot be closed and the reactor continues to produce full steam power.
In that event, and in a boiling water reactor (BWR), a pressure spike occurs, the position of the boiling boundary between the single and two-phase regions within the reactor vessel shifts upwardly, the average void distribution of the fuel changes downwardly and the overall power output increases. This changes the axial power or flux shape of the fuel rod bundle. The axial power or flux shape is the power associated with each axial location in the fuel bundle. In a typical situation, and early in the life of the fuel, the power distribution is at a peak adjacent the bottom of the fuel bundle and the instantaneous power along the fuel bundle upwardly from the peak falls off. When a transient occurs, the boiling boundary moves upwardly along the fuel bundle and displaces the peak of the heat flux curve upwardly along the bundle. The value of the peak of the heat flux curve also changes in response to movement of the boiling boundary. Thus, as the transient progresses, the peak of the heat flux curve is not only displaced upwardly along the fuel bundle but also changes in value so that a greater outlet peak power distribution momentarily occurs. As the effect of control rod insertion and increased reactor voiding decrease neutron flux levels, the peak reactor surface heat flux will be in approximately two seconds.
Present-day nuclear fuel bundle simulators employ fuel rod simulators in a closed vessel containing a coolant. Such fuel bundle simulators, however, are limited in their capability. For example, present testing matches total bundle fuel rod surface heat flux with anticipated nuclear fuel bundle surface heat flux and it is the result of these tests which are used to quantify transient computer codes for application to reactor transient analyses. More particularly, today all test facilities employ heating elements in which the only control over each heating element is the magnitude of the power input to the heating elements. While more or less power could be supplied to the heating elements, the simulated axial power or flux shape is fixed. Thus, while the power supply to the heating elements could be varied over time to obtain the correct total bundle power as a function of time, the change in the axial power or flux shape could not be simulated.